Mardi 26 septembre 2017 à 11h

LATMOS site Guyancourt

Salle de réunion 2202

Pluto's atmosphere: our view after New Horizons par Leslie Young

Southwest Research Institute, Boulder, USA

Pluto's atmosphere is mainly N2 with minor amounts of CH4 and CO, sourced from their surface ices. Photochemistry leads to nitriles (e.g., HCN) and small hydrocarbons (C2H2, C2H4, C2H6), which lead to hazes measured below 200 km. It is expected that KBO dust introduces H2O into the atmosphere. CH4 and N2 escape via enhanced Jean's escape - not by the expected hydrodynamic escape because the low temperatures seen in Pluto's upper atmosphere. HCN, H2O, and C2H6 have all been implicated in the low temperatures seen in Pluto's upper atmosphere. Temperatures in the middle atmosphere are controlled by radiative balance between CH4, CO, and possibly hydrocarbons and H2O. In the lower atmosphere, thermal conduction carries energy down through a very steep thermal gradient from the ~105 K atmosphere to a small boundary layer near 38 K over N2 ice, and somewhat warmer surface over bare areas. Observed clues to atmospheric dynamics on Pluto include the boundary layer, a small eddy diffusion coefficient, homogeneity of composition at the two locations probed by the the New Horizons solar occultation, and the observed latitudinal variation of the haze brightnesss. Global circulation models are used to predict winds. Puzzles remain, some of which can be addressed with ground-based observations.